Remove Checkbox disabled, hover, clicked, etc. graphics and keep only true & false graphics

[Bhavye Mathur]

The Checkbox class currently takes in the true graphic & false graphic as well as hover, clicked, & disabled graphics each for true & false. However, a user can simply supply a Button object for the true & false graphics that handles disabled, clicked, and hover graphics automatically.

Thus there is no need to supply the disabled, hover, & clicked graphics for the Checkbox separately. So remove the unneeded inputs of the checkbox class.